As is well known, conversion of pig iron to steel in open hearth furnaces, top blown basic oxygen converters and bottom blown converters is accompanied by emission of gases and smoke. The particulate matter comprising the smoke includes a significant amount of finely divided iron oxide. It is undesirable to permit the smoke to enter the atmosphere and pollute it. Moreover, the loss of iron oxide from the molten metal within the vessel means that the steel yield is reduced since some of the iron is lost as smoke. In most steel making plants, the particulate matter constituting the smoke is separated from the flue gases in either an electrostatic precipitator or in a wet flue gas scrubber. The gases are variously discharged to the atmosphere, or burned or collected and used as fuel gas since they contain carbon moxoxide which is combustible. The finely divided solids were usually treated as waste.